1. Field of Invention
The present invention relates generally to power conversion electronics and, more particularly, to synchronous rectifier drive circuits.
2. Description of the Background
DC-to-DC power converters are power processing circuits which convert an unregulated input DC voltage to a regulated DC output voltage. Switch-mode DC-to-DC power converters typically include an inverter, a transformer having a primary winding coupled to the inverter, and a rectifying circuit coupled to a secondary winding of the transformer. The inverter typically includes a switching device, such as a field effect transistor (FET), that converts the DC input voltage to an alternating voltage, which is magnetically coupled from the primary winding of the transformer to the secondary winding. The rectifying circuit rectifies the alternating voltage on the secondary winding to generate a desired DC output voltage.
It is known to use synchronous rectifiers (SRs) employing metal-oxide-semiconductor field effect transistors (MOSFETs) to convert the alternating voltage of the secondary winding to the unipolar DC output voltage. The advantage of synchronous rectification is that the forward voltage drop, and hence the power loss, across a MOSFET SR is much less than that of diode devices used in the rectifying circuit. Such SR circuits, however, typically require gate drive circuitry to render the MOSFET at a low resistance during forward conduction and, more importantly, to render it non-conductive during reverse bias. This is because, unlike a diode, a SR may be conductive in both directions (i.e., forward and reverse). Thus, if not properly controlled, reverse current can flow through a MOSFET SR, thereby negatively affecting the efficiency of the power converter.
One known technique to control the gate drive of a MOSFET SR is to couple the alternating voltage from the secondary winding of the transformer to the gate terminal of the MOSFET SR to thereby turn the device on and off in response to the voltage across the secondary winding. This scheme is commonly referred to as xe2x80x9cself-driven synchronous rectification.xe2x80x9d Although usually effective, it is possible that when the voltage on the secondary winding reverses and the gate terminal of the SR is driven off, a delay in turn-off of the SR will provide a period of reverse current in the SR. This has a deleterious xe2x80x9cshortingxe2x80x9d effect on the secondary winding, which may limit the turn off voltage and further delay commutation of the SR. Additionally, it is difficult to generate the proper on-state SR bias level in the self-driven configuration.
Further drawbacks with self-driven SR schemes exist. Self-driven circuits typically do not provide sufficiently fast turn-on and turn-off the SR. Rather, self-driven circuits typically provide slowly rising and slowly falling gate signals that transition the SR through a linear region during which I2R losses are more significant. In addition, self-driven circuits do not achieve optimal timing. That is, for one, the turn-on current is not applied immediately after the SR becomes biased to conduct such that any conduction of the internal body diode of the SR is minimized, thereby reducing losses. This is because self-driven circuits rely on the winding voltage to turn on, and during the rise of current in the SR the winding voltage may be reduced by leakage inductance in the transformer. Also, proper timing of the SR suggests that the gate of the SR be discharged a small delay period before the voltage reverses across the SR. The delay period provides for the turn off time of the SR and ensures that the device is off when reverse bias is applied, preventing any flow of reverse current. Self-driven circuits, however, use the reverse bias voltage itself to initiate turn off and, therefore, no delay is possible. Thus, during the turn off time of the SR, reverse current may flow.
Additionally, self-driven circuits often do not provide a suitable gate voltage to the SR. Ideally, when turning the SR ON, the gate of the SR should receive sufficient voltage to lower the on resistance of the SR to the minimum value. But the gate voltage should not be so high as to damage the gate of the SR. In addition, the source of the voltage for the drive circuit should be referenced to the control terminal (i.e., gate) of the SR and should be able to supply a high pulse current. Self-driven circuits, however, require that the configuration of the SRs be adapted to match the available winding voltage. Further, the pulse current from the windings may be limited by the leakage inductance of the transformer. Furthermore, self-driven circuits apply the winding voltage directly to the gate of the SR. This voltage must be scaled to the converter output voltage, which may be either insufficient or extreme for the gate of the SR.
One known technique to overcome the shortcomings of self-driven synchronous rectifiers is to employ a gate drive circuit coupled to the control terminal of the synchronous rectifier (SR). Gate drive circuits, however, are complicated to implement, thus reducing reliability and increasing cost. Further, conventional gate drive circuits often do not overcome all of the drawbacks identified above for self-driven circuits, such as rapid turn on and turn off, proper timing, suitable gate voltage. In addition, it is difficult to implement a gate drive circuit driven by the alternating voltage of the transformer that is capable of driving two synchronous rectifiers of a dual output power converter or provide the proper bias levels in low voltage output converters.
Accordingly, there exists a need in the art for a SR gate drive circuit that achieves rapid turn on and turn off of the SR so as to reduce, and even obviate, the delay in turn-off of a SR, to thereby minimize, or eliminate, any period of reverse conduction of the SR and the subsequent shorting effect. There further exists a need for a gate drive circuit that is capable of providing the required SR bias level, even for low output converters.
In one general aspect, the present invention is directed to a drive circuit for a synchronous rectifier (SR) for a switch mode power converter. The power converter may include, as switch mode power converters do, a main power transformer and a primary switch for cyclically coupling an input source to the main power transformer. The primary switch may be controlled by a control signal, such as according to a pulse width modulation (PWM) scheme. The SR is for rectifying a voltage across the secondary of the main power transformer.
According to one embodiment, the drive circuit includes turn-on and turn-off switches, a charge pump and a pulse transformer. The turn-on switch is for turning on the SR during the intended time period of forward conduction. The turn-off switch is for turning off the SR. The charge pump is coupled to a secondary winding of the main power transformer and is used to provide drive and a power supply for the turn-on switch. The pulse transformer includes primary and secondary windings, wherein the primary winding is responsive to the control signal supplied to the primary switch and the secondary winding of the pulse transformer is coupled to the control terminal of the turn-off switch. The charge pump shifts the winding voltage to an appropriate reference level for the control terminal of the SR. The number of turns on the secondary winding of the main power transformer can be adapted to match the charge pump output to the requirement of the SR gate. That is, the drive voltage of the charge pump may be higher or lower than the converter output.
As will be apparent from the following description, embodiments of the present invention provide a SR drive circuit that achieves rapid turn on and turn off, with proper timing, and with a suitable voltage level for the SR, with simplicity that increases reliability and decreases cost. For example, the drive circuit of the present invention provides an advantage over prior art self-driven synchronous rectification schemes because it provides a manner for eliminating delay in the turn-off of a synchronous rectifier, thus providing the advantage of eliminating the shorting effect of the secondary winding of the transformer. Embodiments of the present invention also provide the advantage of having a mechanized synchronous rectifier turn-on system operable at, for example, low output voltages.
According to another embodiment, the present invention is directed to a power converter including the drive circuit for a synchronous rectifier. The power converter may be any type of power converter including a synchronous rectifier including but not limited to forward converters, flyback converters, and double ended converters such as, for example half-bridge converters, full-bridge converter and push-pull converters.